High-pressure phase materials such as diamond and cubic boron nitride have an excellent hardness, heat conductivity, transparency and the like, and are widely applied to various tools, optical components, semiconductor materials, electronic components and the like. Such high-pressure phase materials may increasingly gain importance.
Within the high-pressure phase materials, diamond has various excellent properties such as the highest hardness among already-known materials, it functions as an excellent electric insulator, and it has a high heat conductivity of about five times that of copper under a high temperature. Further, diamond has a good light transmittance over a wide range in the wavelengths of ultraviolet, the visible and part of the infrared wavelengths range. Diamond can be brought into a semiconductor state by the addition of an impurity. With such properties, diamond has already been applied as a coating material for a tool surface, as an electronic material, as a heat radiator for a particularly high-output semiconductor laser or LSI, and the like. Further, the application of diamond as a high-temperature semiconductor material, operable in a high-temperature range has also been studied.
While natural diamond was applied to industrial used in the past, artificially synthesized diamond is mainly employed nowadays. FIG. 1 shows relatively large-sized polycrystalline diamond, which has been artificially produced by a vapor-phase synthesis process such as plasma CVD (chemical vapor deposition) or the like, (please refer to "Oyo Butsuri" Vol. 55, No. 7, 1986, pp. 640 to 653, for example in connection with) such polycrystalline diamond, however, it is impossible to attain a sufficiently smooth surface by polishing since polycrystalline diamond layers 2 formed on a substrate 1 are heterogeneous. Therefore, it is necessary to employ for a super-precision tool, a single-crystalline diamond, which is homogeneous in this crystal orientation. Such single crystalline diamonds are also preferred for an optical component or a semiconductor material particularly requiring a smooth surface. Since the natural output of such single-crystalline diamond is extremely scarce, a method for artificially producing single-crystalline diamonds is needed.
At present, artificial synthesis of diamond is implemented by a superhigh-pressure method and the aforementioned vapor-phase synthesis process. As to the superhigh-pressure method, it has been reported that a diamond single crystal of at least 10 ct. (1 carat=200 mg) under a superhigh pressure for maintaining the diamond in a stable state (has been produced refer to "Indiaqua" No. 50, 1988, p. 126, for example). As to the vapor-phase synthesis process on the other hand, it has been confirmed that a single-crystalline layer of diamond is grown on single crystals of natural or artificial diamond, refer to "Journal of Crystal Growth" Vol. 31, 1975 p. 44, for example. Japanese Patent Publication No. 224225/1988 and "Science" Vol. 243, 1989, p. 1047 discloses for growing a single crystal of diamond on a dissimilar substrate said last mentioned. However, this method has not yet succeeded in attaining a diamond single crystal of a suitably large size and good quality while also having a thickness of tens of micrometers.
Cubic boron nitride (CBN) has an extremely high heat conductivity, and a high hardness, although the values for CBN are inferior to those of diamond. While it is difficult for diamond to work a ferrous material which reacts with diamond due to heat generated by a cutting operation, cubic boron nitride is effective as a tool for working such a ferrous material. Further, cubic boron nitride may possibly be applied as a heat radiator for an electronic element of an LSI or the like in the future since cBN is an excellent electrical insulator while having a heat conductivity. In addition, it may be possible to form a p-type or n-type semiconductor material of cubic boron nitride with a doping material. In order to use cubic boron nitride in the field of electronics, it is necessary to develop a technique of forming a single-crystalline thin film.
There has been developed a method of vapor-phase synthesizing a polycrystalline film of cubic boron nitride by plasma CVD or the like. Since a sintered body of cubic boron nitride is more suitable as a tool for working iron and steel materials as compared with that of diamond, polycrystalline substances are mass-produced.
However, the aforementioned conventional methods of synthesizing single crystals of diamond or cubic boron nitride have the following porblems:
As to the superhigh-pressure method of producing a single crystal of diamond, it is difficult to grow a diamond single crystal having a large area since a superhigh-pressure container is restricted in size. On the other hand, an apparatus for vapor-phase synthesis has a large scale size and is costly. When a single-crystalline layer of diamond is grown on a core which is formed by a single-crystalline substrate of natural or artificial diamond by vapor-phase synthesis, a large area cannot by attained since it is difficult to obtain a large-sized single crystal of artificial diamond under the existing circumstances. While artificial diamond can be grown up to a diameter of 10 mm by the superhigh-pressure method, it costs a great deal, and a single crystal of natural diamond is still more costly. If a diamond single crystal is grown on a dissimilar substrate by vapor-phase synthesis, on the other hand, distortion is caused due to differences in the lattice constants and in the thermal expansion coefficients between diamond and the material for the substrate, leading to a significantly defective single crystal. As to the growth of single crystals having different lattice constants, a deviation takes place in the atomic arrangement, to cause a defect called lattice misfit dislocation. Further, differences in thermal expansion coefficients lead to different degrees of shrinkage in response to a temperature reduction from a growth temperature of 700.degree. to 1000.degree. C. to room temperature, whereby distortions are caused. However, it is noted that the combination of high-pressure phase materials, is relatively nonproblematic since the materials are similar in lattice constant to each other such that a difference in the lattice constants of diamond and cubic boron nitride is 1.4%, while the thermal expansion coefficients thereof are also similar to each other.
Although cubic boron nitride is relatively easily synthesized by CVD or the like similarly to diamond, there has not yet been established a technique for readily controlling the crystal states such as, single-crystalline, polycrystalline, and amorphous states for implementing a functional improvement. A single crystal of cubic boron nitride, is indispensable for use in an electronic element particularly where LSI is involved. However, so far, merely a fine single crystal of cBN has been obtained by vapor-phase synthesis. Similarly, cubic boron nitride has not been employed in the field of super-precision working tools, because such tools also require single crystals.
Table 1 shows levels of surface roughness (R.sub.max) required for diamond and cubic boron nitride materials that are intended for the above mentioned uses, and levels after polishing.
TABLE 1 ______________________________________ Level R.sub.max (.ANG.) ______________________________________ Application Required Optical Application (Lenses, etc.) 100 Semiconductor Substrate 50 Semiconductor Device Component 500 Super-Precision Tool 300 Material Polished Diamond Single Crystal 200 Diamond Polycrystal 700 Diamond Sintered Body 2000 CBN Single Crystal 300 CBN Polycrystal 1000 CBN Sintered Body 2000 ______________________________________